The present invention relates generally to methods and apparatus for storage devices, and specifically to improving performance of non-volatile memory devices.
Solid-state memory is ubiquitously used in a variety of electronic systems including, for example, consumer electronic devices (e.g., cellular phones, cameras, computers, etc.) and in enterprise computing systems (e.g., hard drives, random access memory (RAM), etc.). Solid-state memory has gained popularity over mechanical or other memory storage techniques due to latency, throughput, shock resistance, packaging, and other considerations. Among these non-volatile memory devices, NAND flash memory devices are popular due to low manufacturing cost for a high degree of integration.
Data stored in solid-state memory devices may degrade over time. For example, read disturb is a type of degradation that is introduced during the process of reading solid state storage (e.g., NAND Flash). During a read, the control gates of all cells in the selected block (other than the cells being read) are biased to a voltage which exceeds the maximum voltage that any cell in the block may be written to. In doing so, these cells are placed into the pass state, in which they act as short-circuits, thus isolating only the cells which are being read. An undesirable byproduct of this biasing, however, is the application of additional electrons to the floating-gates of these cells. These unintentional and additional charges can change the cell voltage and is referred to as read disturb.
When a large number of read-operations are performed on a flash memory block, the controller may decide to perform background read operations on various pages in that block to determine the “health” of the block, as measured by the number of bit-errors in the page as reported by the ECC decoder. The larger the number of bit-errors, the worse the health of the block. When the number of bit-errors exceeds a pre-determined threshold, the read-reclaimed operation is performed on the block.